1. Field of the Invention
This invention relates to a printed tone control apparatus and a method of determining representative points on prints for controlling tone, which are used in controlling ink feeding rates of a printing machine.
2. Description of the Related Art
In order to perform proper printing with a printing machine, it is necessary to control ink feeding rates properly. For controlling the ink feeding rates, it has been conventional practice to measure densities of control strips with a densitometer and determine from density data whether the ink feeding rates are proper or not. However, the density data from the control strips alone is not necessarily sufficient for attaining a proper color tone and the like for a picture area.
For this reason, a print quality measuring apparatus is used which provides control data for controlling the ink feeding rates of a printing machine. The control data is produced by comparing an image on reference paper (OK-sheet) and an image on an actual print.
The reference paper is also called proof paper, and serves as a reference indicating a color tone of finished prints to obtain proper prints. Printing paper actually printed is also called sampling paper which is extracted by the operator from a discharge station of a printing machine at certain intervals during a printing operation. The printing is considered proper when the color tone on the sampling paper substantially coincides with the color tone on the reference paper.
Further, a print quality measuring apparatus has been proposed, as described in Japanese Unexamined Patent Publication 2001-353852, which compares an image on reference paper and an image on an actual print to produce control data for controlling the ink feeding rates of a printing machine.
Where such a print quality measuring apparatus is used, it is difficult to attain an agreement in color tone and the like in all areas between the image on the reference paper and the image on the actual print. For this reason, use is made of representative points having colors that characterize a picture on the print. These representative points are used in order to perform a color control of the image, and are set to the image by the operator.
The ink feeding rates of a printing machine are controlled for each area corresponding to an ink key of the printing machine. It is therefore necessary to set a representative point for each area corresponding to an ink key of the printing machine. A large printing machine has a large number of ink keys. Thus, the operator must go through a complex operation to set representative points for the numerous areas corresponding to the ink keys.